Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters, such as ladder filters having electrically connected series and shunt resonators formed in a ladder structure. The filters may be included in a duplexer, for example, connected between a single antenna and a receiver and a transmitter for respectively filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW) resonators, including film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), or surface acoustic wave (SAW) resonators. The resonators generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic device comprising a stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). One type of BAW resonator includes a piezoelectric film as the piezoelectric material, which may be referred to as an FBAR as noted above. FBARs resonate at GHz frequencies, and are thus relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns.
Resonators may be used as band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. The passbands of the resonator filters tend to shift in response to environmental and operational factors, such as changes in temperature and/or incident power. For example, the passband of a resonator filter moves lower in frequency in response to rising temperature and higher incident power.
Cellular phones, in particular, are negatively affected by shifts in passband due to fluctuations in temperature and power. For example, a cellular phone includes power amplifiers (PAs) that must be able to deal with larger than expected insertion losses at the edges of the filter (duplexer). As the filter passband shifts down in frequency, e.g., due to rising temperature, the point of maximum absorption of power in the filter, which is designed to be above the passband, moves down into the frequency range at which the majority of the power is emitted from the PA. At this point, the filter begins to absorb more power from the PA and heats up, causing the temperature to increase further. Thus, the filter passband shifts down in frequency more, bringing the maximum filter absorbing point even closer. This sets up a potential runaway situation, which is avoided only by the fact that the reflected power becomes large and the filter eventually settles at some high temperature. One possible solution to mitigate this run-away situation is to stabilize the spectral location of filter's pass-band with respect to temperature changes.
PAs are designed specifically to handle the worst case power handling of the filter at the corner of the pass band. Currents of a typical PA can run from a few mA at the center of the filter passband to about 380 mA-450 mA at the edges. This is a significant power draw on the PA, as well as the battery that drives the cellular phone and is even more pronounced if the filter's maximum absorption point moves towards the frequency range where the RF power emitted from the PA is the largest. This may be one of the reasons that a cellular phone operating more in the transmit mode (i.e., talk time) than in the receive mode (i.e., listening time) drains battery power more quickly. Moreover, power consumption disparity between transmitter PA and receiver PA is because transmitted power needs to reach the base station that may be located at a distance requiring elevated levels of RF power emitted by the cell-phone.
Increased power consumption can result in increased operating temperatures that can adversely impact the operating frequency of acoustic resonators used in filters and other components of RF “front-end,” such as power amplifiers. In addition, frequency down-shift of filter's passband may cause unwanted interference with other passbands designated either by FCC or the government. Such situation occurs e.g. for so-called sliver bands 13 and 30 in the US. In particular, sliver band require filters with are temperature compensated and have a very narrow passbands (of the order of 1-3%). Currently available piezoelectric materials (e.g. aluminum nitride, zinc oxide, scandium-doped aluminum nitride) are generally feasible for designing optimized non temperature compensated filters with passbands approximately 2-3 times wider. Implementing temperature compensation and narrowing the pass-band of the filter to meet the sliver bands requirements generally results in poorly performing resonators (low quality factor Q) and filters (degraded insertion and rejection losses) comprising these resonators.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.